Storage tanks for retaining liquids, particularly flammable or corrosive liquids, typically are provided with pressure relief valves which limit the internal pressure within the tank to a predetermined value for safety. When the pressure inside the tank rises slowly as in normal storage of flammable liquids, such relief valves automatically open at a predetermined pressure value to relieve the excess pressure within the tank by venting to atmosphere. When the pressure inside of the tank is reduced below the predetermined pressure value, the relief valve automatically closes and maintains pressure within the tank below the predetermined value.
During storage, use and transportation of such tanks, overturns of the tank frequently occur, such as a tank falling over or a tanker truck rolling over in an accident. During such overturns, a very sudden liquid surge develops internal pressures within the tank which are of an extremely high value and have a relatively short duration. For example, for some liquids, an overturn can create a sudden liquid surge lasting as little as two milliseconds which can develop internal pressures on the order of fifty pounds per square inch which can persist within the tank for approximately fifty milliseconds.
Such excess surge pressures in a typical storage tank and relief valve assembly can lead to a release of large quantities of liquid from the tank by either blowing the relief valve itself or a hatch, such as a manhole cover, off the tank. Alternatively, if the relief valve assembly and other hatches hold, significant quantities of liquid may be sprayed out of the venting relief valve which can create a gaseous fog in the vicinity of the tank.
Relief valves which address such surge problems are illustrated, for example, in U.S. Pat. Nos. 4,555,041 and 4,763,688, both of said patents being owned by the assignee of the present application. U.S. Pat. No. 4,555,041, which is incorporated herein by reference, discloses an inertia valve which prevents the relief valve from opening for as long as the tank remains overturned. U.S. Pat. No. 4,763,688 provides a snubber surge valve assembly for a relief valve which impedes the rate at which pressure can build within the relief valve upon sudden pressure rise from an overturn or any other sudden pressure surge increase within the tank to increase the time before which the relief valve can vent to atmosphere. Such a snubber surge valve assembly, however, is exposed to the flow of gasses and/or liquids within the tank which can be affected by dirt and debris to prevent proper activation of the snubber surge valve assembly and relief valve.
It therefore would be desirable to provide a relief valve for overturn conditions of a storage tank which momentarily prevents opening of the relief valve upon overturn, but after a short period of time permits normal venting of the relief valve while the tank is still overturned and which is not exposed to gas and/or liquid flow from within the tank.